As is known, ON-OFF valves for compressible fluid media have been provided in the past which are actuated by the pressure of the fluid they control, the valve spool being connected to an actuating piston movable within a cylindrical relief chamber and to a damping piston, usually coaxial with the actuating piston, reciprocable in a cylindrical damping chamber. The actuating piston is subjected to the pressure of the compressible medium controlled by the valve to actuate the valve in response to a reduction in pressure in the aforesaid relief chamber in which the actuating piston reciprocates.
Various constructions of valves of this type are known. They are used as either quick-closing valves serving to shut off sections of a pipeline in the event of dangerous operating conditions arising, such as line breaks or the like, or as quick-opening valves responding to excess pressure. Forces for actuating the valve can be produced by an actuating piston which experiences either the pressure of the flow medium itself or a reduction in such pressure.
Since valves of this type are primarily used for safety purposes, they must be very reliable in operation. It is for this reason that such valves are often of a size and design to insure that the forces which actuate them are much larger than necessary so that the valve operates satisfactorily even when the valve spool or the like has a tendency to stick or becomes difficult to move for any reason once the valve has been in use for a period of time.